1. Field of the Invention
The present invention relates generally to printed circuit boards, and more particularly, to arrangements of different pairs in printed circuit boards.
2. Prior Art
Crosstalk is an omnipresent phenomenon in a high frequency circuits. The existence of crosstalk in high frequency circuits induces unexpected erroneous signals from a driven signal trace to a victim signal trace, the crosstalk screws up the original signals intended to be sent on the victim signal trace. For this reason, efforts have been devoted to develop means for eliminating such unwanted crosstalk phenomena.
One reason for employing differential pairs in high frequency circuits is that crosstalk in differential pairs may largely be reduced. The electronics industry has evolved to demand a higher and higher integration of signal traces in one single printed circuit board, however, since crosstalk is sensitive to the spacing between signal traces, i.e. the larger the spacing, the smaller the crosstalk, crosstalk between two differential pairs is no longer negligible. Referring to FIG. 1, wherein an arrangement of two differential pairs in a printed circuit board 100 is illustrated. As shown, the printed circuit board 100 comprises a dielectric layer 110, a first differential pair 120 comprising a first signal trace 121 and a second signal trace 122, and a second differential pair 130 comprising a third signal trace 131 and a fourth signal trace 132. In this particular example of prior art, the spacing between the first signal trace 121 and the second signal trace 122 is equal to the spacing between the third signal trace 131 and the fourth signal trace 132. Both the first differential pair 120 and the second differential pair 130 are disposed on the dielectric layer 110. Conventionally, the dielectric layer 110 is disposed on a ground plane (not shown). Since the presence of a ground plane does not affect crosstalk between the first differential pair 120 and the second differential pair 130, the illustration of which is thus neglected.
As shown in FIG. 1, suppose that the first differential pair 120 is a driven pair and the second differential pair 130 is a victim pair, the crosstalk on the second differential pair 130 due to the first differential pair 120 is equal to the sum of the crosstalk induced on the third signal trace 131 and on the fourth signal trace 132. The crosstalk induced on the third signal trace 131 and on the fourth signal trace 132 are equal to the sum of crosstalk due to the first signal trace 121 and the second signal trace 122. One may denote the crosstalk on the third signal trace 131 due to the first signal trace 121 by X31, the crosstalk on the third signal trace 131 due to the second trace 122 by X32, the crosstalk on the fourth signal trace 132 due to the first signal trace 131 by X41, and the crosstalk on the fourth signal trace 132 due to the second signal trace 132 by X42. Since crosstalk is primarily induced at the rise time or the fall time of a signal, if a positive crosstalk is induced at the rise time, a negative crosstalk is then induced at the fall time. Therefore, the total crosstalk induced on the second differential pair 130 due to the first differential pair 120 is equal to (X31−X32)−(X41−X42). Since the spacing between the third signal trace 131 and the first signal trace 121 is equal to the spacing between the fourth signal trace 132 and the second signal trace 122, X31 equals X42. Consequently, the total crosstalk induced on the second differential pair 130 due to the first differential pair 120 is equal to (2*X31−X32−X41). In order to make this resultant total crosstalk negligible, one way is to increase the spacing between the first differential pair 120 and the second differential pair 130, leaving X31 approaching X32 and X41 approaching X31 (or X42). By doing so, large real estate of a printed circuit board is required, which greatly limits the integration of signal traces on a printed circuit board. Alternatively, the other way is to set the second differential pair at another layer of the printed circuit board and electrically isolated from the first differential pair.